Aortic dissections and aneurysms: Pathology review

Ronda is a 55 year old with a history of hypertension, diabetes and coronary artery disease who came to the emergency department with a 1 hour history of sudden chest pain that’s described as “something’s tearing inside my chest!” She says this pain is different from the occasional chest pain she gets when she exercises. She was given nitroglycerin in the ER but the pain did not improve. Her blood pressure is 175/95 in the right arm, but 130/80 in the left arm. An ECG shows left axis deviation, but no ST segment changes. Her troponin levels are normal. This is what her chest x-ray looked like. After seeing this x-ray, a CT of the chest was performed, and this is what it looked like.

Okay, so the aorta is subject to a lot of stress, so a lot can go wrong. First let’s look at aortic aneurysms which is an outpouching that occurs due to weakening of the aortic wall. A true aneurysm involves all three vessel layers; the tunica intima, media and adventitia. A dissection occurs when there is a tear in the tunica intima, allowing blood to literally “dissect” into the vessel wall. This may create the appearance of an aneurysm, but because it doesn’t involve all three layers, it’s a false, or pseudoaneurysm.

Now, that we got the basic terminology down, let’s take a closer look at aortic aneurysms. These usually occur in the abdominal aorta, but can also occur in the thoracic aorta. The most high yield and most important risk factor for abdominal aortic aneurysms is atherosclerosis. In atherosclerosis, chronic inflammation results in the release of enzymes called matrix metalloproteinases, or MMPs, which degrade the extracellular matrix in the tunica media, weakening the aortic wall.

Abdominal aortic aneurysms, or AAAs, are most common below the origin of the renal arteries, which corresponds to the L2 vertebral level. This is because below this level, the abdominal aorta lacks “vasa vasorum”, which sounds like a graduation title, but are in fact small blood vessels in the adventitial layer that provide nutrients to the aorta itself. Therefore, the absence of vasa vasorum in this part of the aorta makes the tunica media particularly susceptible to ischemia. That’s because in atherosclerosis, thickening of the intima makes it harder for oxygen to diffuse to the tunica media. Other important risk factors include age greater than 65, male sex, smoking, and family history of abdominal aortic aneurysm.

Now, abdominal aortic aneurysm is usually asymptomatic, but sometimes a pulsatile abdominal mass may be detected on physical exam. The most worrisome complication of AAA is rupture, which is more likely to occur when the aortic diameter exceeds 5 centimeters. It presents with a triad of sudden-onset lower back or flank pain, hypotension from blood loss, and a pulsatile abdominal mass. Your exams will often not give you all three, because in real life it’s rare for the entire triads to appear. Other complications include formation of a thrombus, which could embolize to distal sites like the limbs or the renal arteries.

Thoracic aortic aneurysms are less common, and they’re not caused by atherosclerosis. Rather, they’re associated with cystic medial degeneration. On microscopy, the tunica media appears fragmented and small cleft-shaped spaces appear. Important risk factors you need to remember for your test include hypertension, bicuspid aortic valve, and connective tissue diseases like Marfan syndrome. Also, a high yield fact is that tertiary syphilis can cause a small-vessel vasculitis in the vasa vasorum of the thoracic aorta, also called obliterative endarteritis, which could also lead to aneurysms. Now a thoracic aortic aneurysm involving the ascending aorta can dilate the area where the aortic valve opening is, impairing the valve’s ability to close during diastole. Therefore, you can sometimes hear an aortic regurgitation murmur.

Okay, an aortic dissection is an intimal tear that allows blood to seep into and split the aortic wall, creating a second, false lumen. Risk factors include hypertension, which is the most common cause, bicuspid aortic valve, connective tissue diseases like Marfan syndrome and Ehler-Danlos syndrome, and the use of amphetamines or cocaine. An aortic dissection occurs due to the increased shearing forces against the intimal wall. Additionally, in hypertension, hyaline arteriolosclerosis of the vasa vasorum weakens the aortic wall, making dissection more likely. Aortic dissection classically presents with sudden-onset, tearing chest pain that radiates to the upper back, between the scapula. On physical exam, the blood pressure in both arms may be unequal, and an aortic regurgitation murmur may be heard. A chest x-ray shows widening of the mediastinum, which is a helpful clue, but is not a specific sign for aortic dissection.